Phospholipids are major components of cell and organelle membranes, blood lipoproteins, and lung surfactant. In terms of pulmonary drug delivery, phospholipids have been investigated as therapeutic agents for the treatment of respiratory distress syndrome (i.e. exogenous lung surfactants), and as suitable excipients for the delivery of actives. The interaction of phospholipids with water is critical to the formation, maintenance, and function of each of these important biological complexes (McIntosh and Magid). At low temperatures in the gel phase, the acyl chains are in a conformationally well-ordered state, essentially in the all-trans configuration. At higher temperatures, above the chain melting temperature, this chain order is lost, owing to an increase in gauche conformer content (Seddon and Cevc).
Several exogenous lung surfactants have been marketed and include products derived from bovine lungs (Survanta®, Abbott Laboratories), porcine lungs (CuroSurf®, Dey Laboratories), or completely synthetic surfactants with no apoproteins (e.g. ALEC®, ExoSurf® Glaxo Wellcome). To date, these products have been utilized for the treatment of infant respiratory distress syndrome (IRDS). None have been successful in receiving FDA approval for the treatment of adult respiratory distress syndrome (ARDS). The current infant dose is 100 mg/kg. For a 50 kg adult, this would translate into a dose of 5 g. A dose of this amount can only be administered to ARDS patients by direct instillation into the patient's endotracheal tube, or possibly via nebulization of aqueous dispersions of the surfactant material.
Instillation of surfactants leads to deposition primarily in the central airways, and little of the drug makes it to the alveoli, where it is needed to improve gas exchange in these critically ill patients. Nebulization of surfactant may allow for greater peripheral delivery, but is plagued by the fact that (a) current nebulizers are inefficient devices and only ca. 10% of the drug actually reaches the patients lungs; (b) the surfactant solutions foam during the nebulization process, leading to complications and further loss of drug. It is believed that as much as 99% of the administered surfactant may be wasted due to poor delivery to the patient. If more effective delivery of surfactant could be achieved, it is likely that the administered dose and cost for treatment of ARDS could be dramatically decreased.
Further, lung surfactant has been shown to modulate mucous transport in airways. In this regard, the chronic administration of surfactant for the treatment of patients with chronic obstructive pulmonary disease (COPD) has been suggested. Still other indications with significantly lower doses may be open to treatment if a dry powder form of a lung surfactant were available. The powdered surfactant formulation may be purely synthetic (i.e. with no added apoproteins). Alternatively, the powder formulation could contain the hydrophobic apoproteins SP-B or SP-C or alternative recombinant or synthetic peptide mimetics (e.g. KL4).
Due to its spreading characteristics on lung epithelia, surfactant has been proposed as the ideal carrier for delivery of drugs to the lung, and via the lung to the systemic circulation. Once again, achieving efficient delivery to the lung is important, especially in light of the potential high cost of many of the current products. One potential way to deliver drugs in phospholipids is as a dry powder aerosolized to the lung. Most fine powders (<5 μm) exhibit poor dispersibility. This can be problematic when attempting to deliver, aerosolize, and/or package the powders.
The major forces that control particle-particle interactions can be divided into short and long range forces. Long-range forces include gravitational attractive forces and electrostatics, where the interaction varies as the square of the separation distance. Short-range attractive forces dominate for dry powders and include van der Waals interactions, hydrogen bonding, and liquid bridging. Liquid bridging occurs when water molecules are able to irreversibly bind particles together.
Phospholipids are especially difficult to formulate as dry powders as their low gel to liquid crystal transition temperature (Tm) values and amorphous nature lead to powders which are very sticky and difficult to deaggregate and aerosolize. Phospholipids with Tm values less than 10° C. (e.g. egg PC or any unsaturated lipids) form highly cohesive powders following spray-drying. Inspection of the powders via scanning electron microscopy reveals highly agglomerated particles with surfaces that appear to have been melted/annealed. Formulating phospholipid powders which have low Tm are problematic, especially if one hopes to achieve a certain particle morphology, as in the case of aerosol delivery. Thus, it would be advantageous to find ways to elevate the Tm of these lipids. Examples of particulate compositions incorporating a surfactant are disclosed in PCT publications WO 99/16419, WO 99/38493, WO 99/66903, WO 00/10541, and U.S. Pat. No. 5,855,913, which are hereby incorporated in their entirety by reference.
Currently, lung surfactant is given to patients by intubating them and instilling a suspension of lung surfactant directly into the lungs. This is a highly invasive procedure which generally is not performed on conscious patients, and as do most procedures, carries its own risks. Potential applications for lung surfactant beyond the current indication of respiratory distress syndrome in neonates are greatly limited by this method of administration. For example, lung surfactant may be useful in a variety of disease states that are, in part, due to decreased lung surfactant being present in the lungs. U.S. Pat. Nos. 5,451,569, 5,698,537, and 5,925,337, and PCT publications WO 97/26863 and WO 00/27360, for example, disclose the pulmonary administration of lung surfactant to treat various conditions, the disclosures of which are hereby incorporated in their entirety by reference. Diseases that are thought to be possibly aggravated by lung surfactant deficiency include cystic fibrosis, chronic obstructive pulmonary disease, and asthma, just to name a few. The delivery of exogenous lung surfactant, in a topical fashion, to patients suffering from these diseases may ameliorate certain signs and symptoms of the diseases. For chronic conditions, the regular (once or more times per day on a prolonged basis) delivery of lung surfactant via intubation and instillation to ambulatory patients is impractical. Further, because of their high surface activity, lung surfactant suspensions are not amenable to nebulization due to foaming. The current delivery of phospholipid-based preparations by instillation or nebulization are highly inefficient in delivering material to the peripheral lung. Therefore, the ability to deliver lung surfactant to patients via dry powder inhalation would be a tremendous advantage over the current method, since it would avoid the need for intubation, thereby expanding the potential uses of lung surfactant in the clinical setting.